What are the +/-'s of multiple servers (openvz)

So I am curious what benifits or problems come from a multiserver setup on one server in multiple containers? The first thing that comes to mind is security, and being able to lock down unnecesarry ports and programs to minimize holes in the net. The second would be something like load-balancing or server load can be managed better because high volume servers like web servers or db servers can be given additional resources while the bare minimum for something like a name server (or something to that effect.. I don't know if name servers really need much or not..)

I am mainly wondering if I really need to be separating my email and web servers, or if I am wasting an IP addy and resources because I have dedicated web and mail servers.